The coordination of angiogenesis with the acquisition of specific properties of endothelial cells that line the blood vessels in distinct organs is essenial for their proper function. An important example of this principle is found in the brain endothelium, namely the blood-brain barrier (BBB), that restricts paracellular diffusion of molecules into the brain by forming high resistance tight junctions between endothelial cells. Despite its importance for the central nervous system (CNS) function, the mechanisms that regulate angiogenesis and development of this barrier remain poorly characterized. We have previously identified that Wnt/?-catenin signaling is active in brain but not liver endothelium during CNS angiogenesis and barrier formation. Moreover, this pathway is essential for CNS angiogenesis and acquisition of some barrier properties by endothelial cells. In addition, Apcdd1, a downstream effector of Wnt/?-catenin signaling, is highly expressed in CNS endothelial cells after angiogenesis when endothelial cells acquire BBB properties. Apcdd1 is selectively expressed in CNS, but not, peripheral endothelial cells. Apccd1 is present in CNS endothelium until postnatal day 20 (P20), but it is extinguished in the adult CNS blood vessels when angiogenesis is complete and BBB is fully formed. The protein is localized within the plasma membrane and the secretory pathway and it inhibits the activation of Wnt/?-catenin signaling in a cell-autonomous manner when overexpressed in cells that receive Wnt signaling. We propose that Apcdd1 inhibits Wnt/?-catenin signaling in CNS endothelium to allow cells to mature and acquire BBB properties. In this proposal, we will investigate the role of Apcdd1 in CNS angiogenesis and BBB formation. We will first examine if Apcdd1 interacts with Wnt ligands (e.g. Wnt7a/7b) that induce barrier properties in endothelial cells. Then we will test if Apcdd1 is necessary and sufficient to induce various aspects of angiogenesis and barrier properties in endothelial cells in vitro. We have generated Apcdd1 knockout mice using gene targeting methods and mice that overexpress Apcdd1 in CNS endothelial cells in an inducible manner. These mice will allow us to test the requirement and sufficiency of Apcdd1 for CNS angiogenesis and BBB formation in vivo. Understanding the development of CNS angiogenesis and BBB will shed light on the mechanisms of tight junction formation within CNS endothelial cells, the etiology of pathological CNS conditions associated with abnormal CNS angiogenesis and BBB breakdown, and help to develop novel therapeutic approaches to regulate these processes.